Ballistic Impact Behavior of Novel Coextruded Polycarbonate/Polymethyl Methacrylate Multilayers

Ballistic impact behavior of coextruded polycarbonate (PC)/polymethyl methacrylate (PMMA) composites consisting of 256, 1024, 2048, and 4096 layers with various composition has been evaluated. The individual layer thickness of PMMA was determined to be critical for the impact response of these multilayers. A brittle/ductile transition occurred as evidenced by a significant increase in the damage zone size when the PMMA layer thickness was reduced to approximately 0.3 to 0.4 microns; a mixed mode of failure resulted in these composites. With the PMMA layer thickness further reduced to approximately 0.15 microns, ductile deformation, which was predominant in the PC control, occurred in the PC/PMMA multilayers, and the resulted damage zone was limited to the immediate vicinity of impact A brittle mode of failure, however, was encountered in all the PC/PMMA composites as the thickness of PMMA layers reached 0.5 microns or higher, regardless of their composition and layer configuration. Results of the ballistic impact energy measurements also revealed that the PMMA layer thickness was the critical parameter, and the determined impact energy values were consistently higher for the multilayers with PMMA layer thickness being 0.15 microns or thinner. In addition, microcracking in PMMA appeared to be the dominant mode of failure. Stress concentration associated with these microcracks in PMMA appeared to be insignificant to promote crack propagation across the ductile PC layers when the layer thickness of PMMA is below its critical value.